qiè mò
you must not
Please don't...
be sure not to
on no account (do it)
jiā chǒu bù kě wài chuán , liú yán qiē mò qīng xìn
Don't spread abroad the shame of the family, don't believe rumors lightly (common expression); Don't wash your dirty linen in public, don't listen to others' gossip.